For various reasons, systems have been designed to vaporize fuel oil in the exhaust system of internal combustion engines. One such system relates to onboard smoke generating systems for military vehicles such as tanks and the like.
In these systems, a portion of the engine diesel fuel in injected into the exhaust manifold where the fuel is vaporized and where the vaporized fuel exits with the exhaust gasses. As this vapor is cooled by ambient air as it exits the exhaust pipe, a dense smoke screen is provided that obscures the tank or other vehicle from visual observation.
When such a system is turned off, the residual fuel remaining in the nozzle tubes is vaporized because the temperature in the engine compartment is normally above the boiling point of the fuel. Once the volatile portion of the fuel is gone, a residue remains which is comprised primarily of carbon. There is a gradual build up of carbon after each activation of the system and build-up is referred to as "coking". The build-up of coke is a gradual process and continues until the nozzle tubes become partially or completely blocked. These nozzle tubes, which are normally manufactured from stainless steel, must then be replaced in order to return the system to operational readiness. Maintenance procedures require inspection of the tubes on a regular basis where replacement is made when blockage is found. Of course, frequent replacement involves an unnecessary cost whereas if the system tubes are changed only when the system fails to generate smoke, that blockage would prevent smoke generation which may occur at a time when visual screening is important and possibly even necessary for survival.
As practiced, the visual inspection is undesirable because it is a go, no-go situation since equipment necessary to determine the flow rate through nozzle tubes cannot be provided in the field. While it has not been heretofore possible, elimination of the inspection and replacement maintenance procedure would be most desirable. Particularly desirable would be a system which would prevent the coking build-up in the nozzle tubes to thereby eliminate the need for inspection.